Q: Why is it that when a previous contact in my address book updates their iPhone and then text me they show up as an iMessage under an email address they have instead of their phone number?
A: This actually has nothing to do with your own address book, but is instead a problem on the other person’s iPhone. iMessage can be used with a phone number or an email address, and iPhone users who have signed in with an Apple ID generally have both options available and can choose which one is used as their “From” address by going into Settings, Messages.
While the phone number is normally used by default, it needs to be registered with Apple’s servers when initially configuring or updating an iPhone. In some cases, if this registration doesn’t occur right away, the Messages app may default to using the person’s email address instead, in which case all of their messages will start appearing to come from that address.
To make matters even more confusing, unlike Messages on the Mac, the iOS Messages app doesn’t consolidate iMessage conversations from different addresses, even if they’re assigned to the same contact record. Further, once a conversation has been started from an email address, that conversation will continue to use that address, even if the user changes their default sender address in their iOS Messages settings. In short, much like many email clients, Messages always sends replies from the address the the original message was sent to, regardless of your default address settings.
This means you may actually see a new, separate conversation thread when the other person starts doing this, even if they are in your contacts under the same name. If you reply in that new separate conversation thread, you’ll continue using the email address to communicate with the person rather than their phone number. To force it to go back to their phone number, you either need to send a new message to their phone number, or switch back to an older conversation that is using the phone number.
That said, continuing the conversation with an email address instead of a phone number will work just fine, but you will lose the ability to fall back to traditional SMS messages should the other person be unreachable via iMessage.